Fig. 1. Schematic diagrams of (a) Young’s equation, (b) Wenzel state, and (c) Cassie state.

Fig. 2. Comparison of wetting behaviors between (a) superhydrophobic coating and (b) bare Mg substrate.

Fig. 3. Schematically illustrating methods to develop a superhydrophobic coating.

Fig. 4. SEM images of the sample surfaces (a) before and after immersing in 5 wt.% NaCl aqueous solutions for (b) 30, (c) 180, and (d) 1440 min. (e) Schematic diagrams of the corrosion mechanism of the superhydrophobic surface formed on magnesium alloy [63].

Fig. 5. (a) Schematic representation of the experimental setup. (b) Digital images of the large-area superhydrophobic surface. Potentiodynamic polarization curves of the untreated and superhydrophobic AZ61 Mg alloy surfaces in different corrosive solutions: (c) 3.5 wt.% NaCl aqueous solution, (d) 3.5 wt.% Na2SO4 aqueous solution, (e) 3.5 wt.% NaClO3 aqueous solution, and (f) 3.5 wt.% NaNO3 aqueous solution [66].

Fig. 6. (a) Schematic representation of the electroplate. (b) The Nyquist plots obtained from the untreated magnesium alloy substrate, the hydrophobic surface, and the superhydrophobic surface after immersion in 5 wt.% NaCl aqueous solution for 5 min [67]. (c) Schematic diagram of the abrasion test and contact angle of the superhydrophobic surface as a function of abrasion length [72]. (d) Bode plots of the superhydrophobic surfaces with different immersion times in neutral 3.5 wt.% NaCl solution [68].

Fig. 7. SEM images of surface morphology for the (a) Ni coating [68], (b) CuO film [71], (c) Cerium oxide film [72], and (d) Ni-P coating [74].

Fig. 8. EIS and fitted results for an uncoated AZ31 Mg alloy, a MAO coating, and a H-MAO coating in a 3.5 wt.% NaCl aqueous solution: (a) Nyquist plots, (b) Bode plots of |Z| vs. frequency, and (c) Bode plots of phase angle vs. frequency. Photographs of samples after immersion in a 3.5 wt.% NaCl aqueous solution for different times: (d) uncoated AZ31 Mg alloy after immersion for 3 days, and (e) MAO and (f) H-MAO coatings after immersion for 11 days with clearing corrosion products [92].

Fig. 9. SEM morphologies and contact angle (inset) of the (a) MAO coating, (b) MAO/SA 0.5 h, (c) MAO/SA 1 h, (d) MAO/SA 3 h, (e) MAO/SA 7 h coatings. (f) Cross-sectional SEM image of the MAO/SA 7 h coating [93].

Fig. 10. Hydrogen evolution reactions as a function of immersion time for the (a) Mg-4Li-1Ca substrate, (b) MAO, and (c) MAO/ZnSA coating in 3.5 wt% NaCl for 85 h [94].

Fig. 11. Schematic preparation of superhydrophobic coating on MAO pretreated Mg alloy by cyclic assembly of phytic acid and cerium ions [96].

Fig. 12. Schematic representation of the corrosion and protection mechanisms of (a) Mg-(PA@Ce)3-FAS coating, (b) MAO layer and (c) MAO-(PA@Ce)3-FAS coating on Mg alloy [96].

Fig. 13. Schematic illustration for the preparation process of superhydrophobic surfaces on light alloy substrates based on hydrothermal or chemical etching treatments [107].

Fig. 14. Surface morphologies of the coating (a) before and (b) after treatment with stearic acid [113].

Fig. 15. Photographs of various samples after 120 h immersion in 3.5 wt.% NaCl solution: (a) bare AZ31 substrate, (b) the hydrothermal treated sample, and (c) the superhydrophobic sample [113].

Fig. 16. (a) Bode plot of the bare substrate and modified samples, and Nyquist plots for (b) AZ31 substrate, (c) Mg(OH)2/PMTMS coating, Mg(OH)2/PMTMS/CeO21 coating and Mg(OH)2/PMTMS/CeO22 coating, and (d) Mg(OH)2 coating, and Mg(OH)2/PMTMS/CeO23 coating [114].

Fig. 17. Schematic diagram for the preparation of silica-based superhydrophobic coatings on AZ31B Mg alloy sElectrochemical impedance spectra fourfaces [116].

Fig. 18. Schematic illustration of the fabrication process for the durable superhydrophobic coating by spraying method [121].

Fig. 19. EIS of the superhydrophobic coatings immersed in 3.5 wt.% NaCl solution for 0.5 h, 168 h, 336 h and the bare Mg alloy sheets: (a) Nyquist plots, (b) Bode modulus plots, (c) Bode plots of phase angle. Equivalent circuits for (d) the bare Mg substrate and (e) superhydrophobic coating [121].

Fig. 20. Potentiodynamic polarization curves of the uncoated Mg alloys and the superhydrophobic samples in 3.5 wt.% NaCl aqueous solution [121]. EIS results of bare specimen (BS), superhydrophobic film coated specimen (SS) and deaerated SS (DS) in 3.5 wt.% NaCl solution: (b) Nyquist plots and (c) Bode |Z| versus frequency plots [128].

Fig. 21. (a-c) Schematic illustration of three possible wetting states for superhydrophobic surfaces with hierarchical structures. Electrochemical impedance spectra for: (d) lotus-like, (e) rose-like, and (f) Wenzel-like surfaces [129].

Fig. 22. A Si surface with two-scale structures (micro-pyramids and nanostructures): (a) before abrasion, (b) after abrasion on technicloth paper, and (c) after sand abrasion. Inset: SEM image of sand particle size (～100 μm) [133].

Fig. 23. (a) Photographs of water droplets and contact angles on the superhydrophobic surfaces (Scale bars of 5 mm); change of WCAs for the superhydrophobic coatings as a function of repeated polishing and accelerated weathering cycles [141]. (b) Recovery of topographic features by treatment with deionized water for 1 h and low-pH buffer for 60 s; plot of contact angle versus number of crushing/healing cycles applied [142].

Fig. 24. Schematic illustration of the (a) ion-exchange process during the immersion and (b) tungstate release behavior from the coating samples [152].

Fig. 25. (a) Optical photo of a pitcher plant. (b) Wetting schematic of a slippery liquid-infused porous surface (SLIPS). (c) Schematic diagram of fabricating a slippery surface [174].

Fig. 26. Optical photographs of the bare Mg alloy and the Mg alloys coated with LDH-carbonate, superhydrophobic surfaces, and slippery liquid-infused porous surfaces for the long-term immersion test in a 3.5 wt.% NaCl solution: (a) initial state, (b) after 3 days, (c) after 7 days, and (d) after 15 days [161].